The present invention relates to an injection internal combustion engine of a compression ignition type in which fuel is injected directly into a cavity formed in a piston top surface.
Direct injection internal combustion engines of a compression ignition type, in which the combustion chamber is constructed by forming a recess (which will shortly be referred to as a "cavity") in the piston top surface, are frequently used as large-sized engines because these have such an advantage over similar engines having a swirl chamber or a precombustion chamber in that in the direct injection internal combustion engine there is no connecting hole between the different chambers, and a low compression ratio is allowed, so that frictional loss and accordingly the specific fuel consumption of the engine can be reduced.
In a small-sized engine having a small cylinder diameter, however, the direct injection internal combustion engine of a compression ignition type encounters more problems in the formation of the air-fuel mixture than in the large-sized engine.
In a direct injection internal combustion engine according to the prior art, more specifically, the fuel injection nozzle is arranged at the center of a cavity formed in the top surface of the piston so that it may inject a plurality of fuel sprays radially from its plural injection ports. A swirling flow (or a swirl), which is generated by the port or the like of an intake valve during the suction stroke of the engine, still exists even at the end of the compression stroke, so that the mixture is formed while carrying the fuel spray in the swirling direction in the cavity. The diameters of cavities in common use are within a range of 40 to 70% of that of the piston. Thus, a small-sized engine equipped with a piston having a diameter of 100 mm or less reduces the diameter of the cavity C, and this cavity diameter becomes smaller if the compression ratio is made higher. As a result, the fuel sprays injected radially from the plural injection ports of the fuel injection nozzle FIG. 4 collide upon the inner wall surface of the cavity so that they either stick as a liquid film to the wall surface or reside thereon as coarse droplets, to thereby reduce the effective mixture. This in turn results in the failure of effective combustion to thereby invite a reduction in the output power, a degradation of fuel economy, the generation of smoke, and an increase in the hydrocarbons in the exhaust gases.
In order to prevent the fuel from impinging upon the cavity wall surface, the following methods have been generally used: (a) a method by which the swirling flow formed in the combustion chamber is intensified; (b) a method by which the injection ports of the fuel injection nozzle have their size reduced but are increased in number; or (c) a method by which the compression ratio is increased to raise the pressure (or air density) in the cavity at the fuel injecting timing to thereby reduce the spray penetration of the fuel injection nozzle.
Method (a) raises a problem in that if the swirl ratio is increased in a small-sized engine, the fluid resistance of the intake port is increased so that the air charging efficiency of the engine is degraded.
Method (b) raises problems in that the injection ports are liable to be clogged, and the sprays injected from adjoining injection ports are carried by the swirling flow so as to merge in the vicinity of the inner wall of the cavity, so that an overrich region of fuel is partially established to cause smoking.
Method (c) raises a problem in that there is a limit to the reduction in the clearance between the cylinder head and the piston top surface as it does not contribute to combustion. The limit is so severe, especially for a small-sized engine, as to make it difficult to avoid problems due to the thermal expansion of the engine components and in assembling and adjusting the engine.